PURPOSE: To judge existence of leakage of an evaporation system without leading negative pressure concerning an accident diagnosis device of an evapopurge system in which vapor fuel of an internal combustion engine is absorbed to adsorbent inside a canister,, and the adsorbed fuel is discharged to the intake system of the internal combustion engine under a specified operational condition so as burn it.

CONSTITUTION: An inner pressure control valve 29 is provided on the way of a vapor passage 28 so as to maintain the pressure inside a tank at a constantlerd. Pressure P in a gap between a valve device 29 and a fuel tank 21 is detected by a pressure senosr 31 until the lapse of a specified time passes after start. It is detected by a microcomputer 38, whether the pressure P is in a specified range or not, and when it is in the specified range, it is judged abnormal, so as to turn on a warning lamp 39.